Q. What are the supported version of NFS protocol ? How to identify which being used now ?

A.  NFSv3 is supported with XenServer 6.1,6.2,6.5 and XenServer version 7 supports NFSv3 and NFSv4. If the underlying storage array does not support NFSv4, NFSv3 will be used to mount the share. In order to identify which version is running, run the following command

# cat /proc/mounts | grep vers

Or from xapi run the following command

xe pbd-list sr-name-label=NFS\ Storage\ Repository\ 1 params=device-config
 

This will give NFSv4 for V4 protocol and nothing for NFSV3